1. Field of the Invention
The present invention relates to a vehicle power unit to be mounted on a vehicle, and more particularly to a vehicle power unit for a motorcycle including an internal combustion engine and a power transmission unit including a transmission and an output shaft.
2. Description of the Background Art
A vehicle power unit of the background art is described in JP-U-60-34864, the entirety of which is hereby incorporated by reference. In this type of power unit for mounting on a motorcycle, a transmission case and a crankcase, both of which are cylindrical, are placed side by side and formed integrally with a frame housing. The frame housing is formed integrally with a cylinder block of the engine. A transmission member, including a main shaft and a counter shaft, is inserted through an opening end of the transmission case, and one of the ends of the transmission member is journaled by a disc-shaped end lid detachably secured on the opening end by a screw. A clutch is mounted on the projected portion of the main shaft projecting from the end lid.
In the aforementioned arrangements of the background art, an output shaft is connected to the counter shaft via an input shaft. The input shaft engages with the output gear provided on the counter shaft in order to transmit a power of the counter shaft to the wheel. The counter shaft and the output shaft separated from each other with a force exerted due to a reaction force of a transmitted power between the input gear and the output gear. A backrush between the output gear and the input gear exceeds a suitable value, which may increase noise caused by engagement and abrasion of the teeth of the output gear and the input gear. Therefore, it is necessary to enhance the connection rigidity of the end lid.
However, when the fastening points are increased by increasing the number of screws for securing the end lid to the transmission case, the number of components increases and the number of assembling steps disadvantageously increase. When the end lid and the transmission case are formed with a fit-in structure (pen cap structure) in addition to being secured with screws and fitted together to increase the fastening rigidity of the end lid, the number of machining processes for forming the fit-in structure and the corresponding costs increase. In addition, since the counter shaft and the output shaft are journaled by different members, it is difficult to ensure accurate mounting positions for both shafts.